In many industrial environments, it is common for deposits of chemical residues or coatings of scale or other undesirable materials to accumulate on surfaces of equipment and buildings during their ordinary use. This problem also may occur in domestic environments. In some instances, the deposits or coatings may be merely unattractive, whereas in other situations, they may interfere with the normal function of the surface on which they have accumulated.
In the past, a variety of means have been used to clean away such deposits, including manual scrubbing with strong detergents or hard abrasive compounds. Alternatively, high pressure cleaning with large volumes of water or another cleaning liquid, sandblasting, or steam cleaning have been used.
One common example of a previously unresolved cleaning problem is the task of cleaning old paint from aluminum surfaces, such as those of an airplane. When used to clean painted aluminum, the known cleaning methods have not provided satisfactory results for a variety of reasons. Aluminum is a relatively soft substance, particularly in thin sheets. Scrubbing with a steel wire brush may fail to produce satisfactory cleaning results because the steel wire is harder than the aluminum, which results in the wire brush penetrating and damaging the aluminum surface by the scrubbing action. In addition, both manual and machine scrubbing are labor intensive. Thus, these means have failed to give satisfactory results in removing old paint and other deposits from aluminum surfaces.
The use of high pressure liquid cleaning methods heretofore known also have not proven to be advantageous because the high volumetric flow requirement presents the problem of disposing of the large quantities of liquid waste run off.
Another problem associated with high flows of cleaning liquid arises from an excessive impact. This impact, which results from the use of high volumetric flow rates at high pressures, may damage the surface or warp the aluminum. Thus, there has been a need to circumvent the effluent discharge problems and the adverse impact effects associated with high volumetric flow rates used in conventional liquid cleaning systems.
Fluid suspended abrasives also have been used in the past to clean coatings and other deposits from underlying surfaces. The propelling fluid used is generally a gas such as air and the abrasive may be particles of sand, salt, sodium bicarbonate, potassium bicarbonate, and the like. A liquid, such as water, has sometimes been added in relatively small amounts (e.g. less than 0.5 gpm) to reduce the dust generated by the blasting operation when the fine abrasive particles are suspended in the gaseous propellant. Although high pressure water has been injected into the gaseous stream for this purpose, the volume of the injected water relative to the volume of the propelling air has been too small to add much kinetic energy (velocity) to the particulates carried by the air.